The present invention relates in general to pattern defect inspection and foreign material inspection for detecting defects (short circuits, breaking of wire, etc.) in a circuit pattern and foreign materials on a sample; and, more particularly, the invention relates to a method and apparatus for inspecting for defects and foreign materials on a circuit pattern, such as those which appear in semiconductor wafers, liquid crystal displays, photomasks, etc. In the following description, a defect includes the presence of foreign material.
Conventionally, as an example of an inspection apparatus of this kind, there is a technology in which an image of a sample is picked up using an imager, such as an image sensor, while the sample is being moved, and gray scales of the detected image signal and of an image signal that has been delayed for a fixed time are compared, whereby a portion where the two signals do not coincide is identified as a defect (JP-A No. 212708/S61).
Moreover, as another example of technology that is related to defect inspection of a sample, there is a technique for performing high-accuracy inspection on a semiconductor wafer, such that an area of high pattern density, such as a memory mat part, and an area of low pattern density, such as a peripheral circuit, are mixed within the same die (JP-A No. 320294/1996).
Furthermore, as a way of detecting an ultra-fine circuit pattern, there is a technique for detecting the circuit pattern with high resolution, while illumination light or its diffracted light is controlled optically (JP-A No. 318326/1997 and JP-A No. 155099/2000).
In addition, among techniques for inspecting the circuit pattern of a photomask, there is a method in which UV (Ultra Violet) laser light, such as that of an excimer laser, is used as a light source, its coherence is reduced by rotating a diffuser inserted in the light path and irradiated onto a mask for uniform illumination, and the quality of the photomask is determined by calculating feature quantities from obtained image data of the mask (JP-A No. 78668/1998).
Moreover, as an example of inspection using laser light in the range of UV to DUV (Deep UV), there is a method of reducing a laser's coherence by swinging the laser light (JP-A No. 194323/2001).
In the LSI manufacture of recent years, the pattern width formed on a wafer has been reduced to 200 nm or less because of miniaturization of the circuit pattern in response to a demand for high integration, and, accordingly, the dimensions of defects to be detected also have become ultra-fine. In particular, defects have been reported that are difficult for the conventional technology to detect, such as those referred to as “Non Visual Defects.”
Under such circumstances, technologies for realizing a higher NA (Numerical Aperture) in an objective lens to be used for inspection and super resolution are being developed. Since making the NA of the objective lens for inspection higher has already reached the physical limit, it is an essential approach to make the wavelength of illumination light used for detection shorter, i.e., toward the areas of UV (Ultra Violet) light and DUV (Deep UV) light.
LSI devices are becoming complicated and diversified in connection with the structures of inspection target patterns, such as in memory products formed mainly with repeating patterns and in logic products formed mainly with non-repeating patterns, and, consequently, it has become difficult to find with certainty target defects that require product control at the time of the manufacture of LSI devices. FIG. 2 shows an example of various target defects. Target defects that are desired to be detected are voids and scratches produced in a CMP process, in addition to the presence of foreign materials and pattern defects produced in each process. Moreover, there are short circuits and bridges in gate wiring in metal wiring parts, such as parts made of aluminum (Al), as well as non-conduction and non-aperture conditions of contact holes connecting the wiring.
Because underlayer patterns at locations where defects have occurred have become diversified, and due to the fact that the shape of a defect itself has been minimized (to dimensions equal to or less than the resolution of the conventional optical system) and diversified, it is difficult to detect many defects. Here, as factors that impede detection of target defects, there are grains produced in the metal-wiring processes, such as grains of Al, and a minute unevenness called a morphology. Moreover, in a process where a transparent film (here, meaning transparent to the illumination light), such as insulator film, is exposed at the outermost surface, an intensity nonuniformity in interference light due to minute film thickness differences of the transparent film appear as optical noise. Therefore, there is a problem in that target defects whose dimensions are equal to or less than the resolution of the optical system are made apparent, while erroneous detection of a grain and morphology is lessened, and the influence of the intensity nonuniformity in interference light is reduced.
There is a method of reducing the above-mentioned intensity nonuniformity in interference light by using lamps as a light source, for which light of a plurality of wavelengths is irradiated. However, if the light of a plurality of wavelengths is obtained from one light source, the ratio of wavelength intensities changes in accordance with the time of use of the lamp. FIG. 3 is a graph showing the output intensity of a certain lamp over time. FIG. 3 shows the output intensity 51 of a wavelength λ1 and the output intensity 52 of another wavelength λ2. At time A, the intensity ratio of the output intensity 51 to the output intensity 52 is about 3, but at time B the intensity ratio becomes 7.5. This is because the attenuation rate over time of use is different for each wavelength. If the intensity ratio is changed in this way, the detected optical image will change in each measurement; therefore, there is a problem in that defect detection cannot be carried out stably.